The catalyst compounds and systems of the present invention are useful in the production of ketones which are important industrial commodity chemicals. For example, methyl ethyl ketone and methyl isobutyl ketone find use as solvents. Further the present invention can be used to make heretofore unavailable ketones which can serve as new classes of useful specialty chemical products, or intermediates used in their production.
Palladium catalysts are useful in the oxidation of unsaturated hydrocarbons. One large class of hydrocarbons is olefins. Depending on the catalyst composition and reaction conditions, a number of different major reaction products may result. The generalized examples are: ##STR1##
A good summary of palladium catalyzed olefin oxidations can be found in Chapter 7 of "Metal-Catalyzed Oxidations of Organic Compounds" by Sheldon and Kochi (Academic Press, N.Y., 1981). A more specific review, "Synthetic Applications of the Palladium Catalyzed Oxidation of Olefins to Ketones" has been written by J. Tsuji; Synthesis 5, 369-384 (1984).
In the oxidations outlined above, Pd.sup.+2 is reduced. The overall reactions can be made catalytic if the palladium can be reoxidized by an oxidizing agent. Preferrentially, one would use plentiful and cheap oxygen from air. The direct reoxidation of palladium by oxygen is thermodynamically possible but kinetically too slow. As a result, a co-catalyst is required to speed up the overall oxidation process.
The Wacker-type oxidation process of the prior art uses PdCl.sub.2 /CuCl.sub.2 as the catalyst system where Cu.sup.+2 plays the role of the co-catalyst. EQU Pd.degree.+2CuCl.sub.4.sup.-2 .fwdarw.PdCl.sub.4.sup.-2 +2CuCl.sub.2.sup.-( 5) EQU 2Cl.sup.- +2HCl+1/2O.sub.2 +2CuCl.sub.2.sup.- .fwdarw.2CuCl.sub.4.sup.-2 +H.sub.2 O (6)
It should be noted that the copper is necessary to improve palladium reoxidation kinetics. Chloride (Cl.sup.-) is an essential ingredient since as a Pd.sup.+2 ligand, it provides a driving force for reaction (5) and, as a Cu.sup.+2 ligand, it makes reaction (6) possible.
The above Wacker system, however, presents several substantial engineering problems making commercial application difficult. The use of chlorides results in severe corrosion, requiring the use of expensive, i.e. titanium-clad, reactor vessels. Further, the presence of chloride ions results in the formation of undesirable chlorinated byproducts which lowers the overall yield of desired material. In addition, these chlorinated by-products often prove difficult to separate from the desired product.
In response to these unfavorable characteristics of Wacker-type catalysts, new systems have been developed by others to reduce the level of chloride present in the olefin oxidation system. The best examples of these newly developed systems can be found in Belgian Pat. No. 828,603, the work of Ogawa et al., J.C.S. Chem. Comm, 1274-75 (1981), and U.S. Pat. No. 4,434,082.
Belgian Pat. No. 828,603 (Oct. 30, 1975) teaches the use of polyoxoanions as co-catalysts to regenerate Pd.sup.+2. The reduced polyoxoanions are subsequently reoxidized with oxygen. Such polyoxoanions can be generally described in the following way.
In aqueous solution certain metal oxides undergo stepwise hydrolysis-oligomerization reactions upon acidification according to the following representative stoichiometries ["Heteropoly and Isopoly Oxometalates" by M. T. Pope (Springer-Verlag, N.Y., 1983)]: EQU 2aH.sup.+ +bMO.sub.n.sup.-r .fwdarw.[M.sub.b O.sub.y ].sup.-p +aH.sub.2 O (7)
where
bn=y+a (oxygen atom balance) PA1 br-2a=p (charge balance) EQU 2aH.sup.+ +bMO.sub.n.sup.-r +cXO.sub.q.sup.-s .fwdarw.[X.sub.c M.sub.b O.sub.y ].sup.-p +aH.sub.2 O (8) PA1 bn+cq=y+a (oxygen atom balance) PA1 br+cs-2a=p (charge balance)
where
and where M can be one of several metals, e.g. W, Mo, V, or mixtures of these metals. X is usually P or Si but can be a number of other elements. The condensed metal oxides, e.g. [X.sub.c M.sub.b O.sub.y ].sup.-p, form a symmetric three dimensional array whose structure and composition can vary a great deal with various X's and M's. Which structure is present depends on the acidity of the solution, the initial amounts of MO.sub.n.sup.-r and XO.sub.q.sup.-s, and other reaction conditions. In some cases, even under the same reaction conditions, different structures may be present. Products formed by reaction (7) are called isopolyoxoanions. Products formed by reaction (8) contain a "hetero" atom X, usually centrally located in the structure, and as a result these products are referred to as heteropolyoxoanions. Hereinafter, polyoxoanion (POA) may be used to refer to heteropolyoxoanions and isopolyoxoanions. Those skilled in the art would be capable of differentiating heteropolyoxoanions from isopolyoxoanions when necessary for clarity.
The Belgian patent discloses a number of heteropolyoxoanion compositions, mostly containing mixtures of molybdenum and vanadium, useful in the oxidation of ethylene to acetaldehyde, propylene to acetone, butene to methyl ethyl ketone and 1-hexene to 2-hexanone. It is also disclosed that isopolyoxoanion compositions can lead to unstable catalyst systems. Further, it is disclosed that an increase in the number of vanadium atoms from one to six is observed to cause an increase in the beneficial characteristics of the catalyst, which catalyst can be prepared in situ without isolation.
High selectivity is predicted for a large number of olefins but only shown for C.sub.2 to C.sub.4 in which cases isomerization either cannot occur (C.sub.2), occurs to give the same structure (C.sub.3), or occurs to give different isomers which react to the same product (C.sub.4) as shown in equation (9). ##STR2##
The examples disclosed in the Belgian patent show that when 1-hexene is used, selectivity to 2-hexanone drops significantly due to isomerization.
The iso- and heteropolyoxoanions of the instant invention, e.g. PMo.sub.6 V.sub.6 O.sub.40.sup.-9, PW.sub.6 Mo.sub.6 O.sub.40.sup.-3, P.sub.2 W.sub.15 V.sub.3 O.sub.62.sup.-9, are used in conjunction with a redox active metal and (or) a ligand for the redox active metal and (or) the palladium component. The addition of the redox active metal component and the ligand, either singly or in combination, results in greatly improved conversions and selectivities not taught by the prior art.
The Belgian patent teaches the use of the polyoxoanion component and the palladium component in ratios of 100:1-1000:1, which leads to very high POA loadings. Lower ratios (2:1 and 33.3:1) require high palladium concentrations. The instant invention reduces the amount of isopolyoxoanion or heteropolyoxoanion required such that favorable catalyst activity is observed when polyoxoanion:Pd ratios of 0.5:1-10:1 are used. The disparity in cocatalyst (POA) loading between the Belgian patent and the instant invention is partially attributable to the fact that the overall oxidation process disclosed in the Belgian patent requires two stages.
In the first stage of that process, the palladium component oxidizes the olefin. In order to achieve commercially acceptable turnovers of the olefin on palladium, large molar amounts of polyoxoanions are required. This results from the fact that the reaction is stoichiometric in the polyoxoanion due to the absence of molecular oxygen. As a result of the high molecular weight of these compounds, large masses of these compounds are correspondingly loaded, rendering commercial operation impractical from solubility, viscosity and catalyst distribution standpoints. Once all the polyoxoanion is reduced, the palladium can precipitate out as the metal (zero valent state). In the second stage, after removal of hydrocarbons, oxygen is added to reoxidize the polyoxoanion.
The favorable catalyst activity of the instant invention enables a one stage oxidation process. While a proper choice of the hydrocarbon/oxidant feed composition and proper reactor design can eliminate potential safety hazards, a one-stage process not only eliminates the need for a second stage, but also eliminates the engineering problems associated with the handling of high viscosity fluids resulting from the use of high polyoxoanion concentrations. The addition of a redox active metal and (or) ligand for the palladium and (or) the redox active metal, not only further reduces the amount of heteropolyoxoanion needed, but in a number of cases these additives produce active polyoxoanion systems from otherwise inactive ones, e.g., P.sub.2 W.sub.12 Mo.sub.6 O.sub.62.sup.-6, which by itself does not reoxidize with oxygen. In addition, the presence of a redox active metal and (or) ligand, unexpectedly increases the olefin oxidation rate and also improves the selectivity and yield to the desired carbonyl product. Furthermore, by using less polyoxoanion, the cost of the catalyst per unit of hydrocarbon product is reduced substantially.
The Belgian patent teaches the use of PdCl.sub.2 and PdSO.sub.4. Although the chloride levels are greatly reduced or supposedly eliminated as compared to the PdCl.sub.2 /CuCl.sub.2 system, the patent further teaches the use of polyethylsiloxane as a corrosion inhibitor. Thus it is obvious that at these high polyoxoanion concentrations, the corrosion problem of Wacker-type systems has merely been mitigated. The catalyst systems of the instant invention do not contain chloride ions except sometimes as eventual trace contaminants introduced during polyoxoanion synthesis. These systems do not significantly corrode commonly used steels, resulting in substantial capital savings in plant construction.
Vanadium-free heteropolyoxoanion compounds useful in olefin oxidations are disclosed in "Liquid Phase Oxidation of Cyclo-olefins by a PdSO.sub.4 -Heteropolyacid Catalyst System" by Ogawa, Fujinami, Taya and Teratani, J.C.S. Chem. Comm., 1274-75 (1981). The catalyst system of interest is PdSO.sub.4 -H.sub.3 PMo.sub.6 W.sub.6 O.sub.40 for the oxidation of cyclohexene to cyclohexanone. Very limited conversions were attained, indicating that the reoxidation of Pd.degree. to Pd.sup.+2 was very inefficient. These systems do not possess commercially viable catalyst lifetimes, especially in view of the high cost of palladium.
The instant invention teaches that the use of a redox active metal component, (and) or a ligand component, in conjunction with the Pd.sup.+2 /H.sub.3 PMo.sub.6 W.sub.6 O.sub.40 system improves the conversion and selectivity. However, in the above mentioned particular case of H.sub.3 PMo.sub.6 W.sub.6 O.sub.40, addition of both a redox active metal component and a ligand increases the oxidation rate by more than two orders of magnitude. This surprising result permits practical application of this catalyst system in an industrial process.
If the redox active metal component is copper, then selectivity to the carbonyl reaction product is greatly improved while copper inhibits the allylic oxidation pathway. This is important in the case of those olefins that have reactive allylic positions, e.g. cyclohexene: ##STR3## U.S. Pat. No. 4,434,082 (Feb. 28, 1984) (hereinafter '082) teaches a Pd.sup.+2 -heteropolyoxoanion-surfactant system useful in olefin oxidation to ketones. A two phase system is employed consisting of an aqueous phase and a hydrocarbon phase. In such a system, the olefin tends to stay in the hydrocarbon phase and the catalyst in the aqueous phase. As a result, the yields of oxidized product, as shown in Example 6 of the '082 patent, are below three percent for the oxidation of 1-butene to methyl ethyl ketone in the absence of surfactants. To improve the reaction kinetics, the surfactant component is essential for bringing the catalyst and reactants into intimate contact. The instant invention shows improved conversions and selectivities without the use of this surfactant component, identified as essential in the '082 patent.
In sharp contrast to the catalysts of the prior art, the use of the catalyst systems of the instant invention results in a more efficient oxidation process from several important process engineering perspectives. The conversion and selectivity to the desired carbonyl product are greatly improved over earlier systems wherein polyoxoanions were used. Catalyst lifetimes are also enhanced in the present systems. This permits the use of less catalyst, resulting in significant savings. Additionally, the present catalyst systems can be used in a single stage oxidation process, reducing process costs for the energy required to pump and heat the reactants and catalysts, as well as capital equipment costs for the second stage process equipment.
The use of chloride-free components eliminates several major engineering shortcomings of the Wacker systems of the prior art. In particular, chloride-free systems exhibit no corrosivity to the process equipment, making the use of stainless steel process equipment possible. This factor improves process economics substantially because initial capital costs for stainless steel equipment are far below those for titanium-clad or glass-lined vessels. Further, chloride-free systems eliminate many of the problems resulting from chloroorganic by-product formation under oxidation process conditions. The separation and disposal of these undesirable chloroorganic compounds present significant engineering and environmental problems when encountered on an industrial process scale.
The present catalyst systems exhibit higher yields than the prior art when more complex substrates are oxidized. Chloride-containing catalysts show a pronounced and rapid dropoff in yield of the desired carbonyl compounds as the number of carbons in the olefin substrate increases. The formation of complex chloroorganic by-products decreases overall yield to the desired carbonyl product. In the present system, the decrease in yield as a function of the increasing number of carbons in the olefin is less pronounced. This allows the economically attractive production of ketones which could not be produced by prior art catalyst systems.
Therefore, it is one object of this invention to provide an efficient catalyst system for olefin oxidation which eliminates the use of corrosive chloride ions.
It is another object of this invention to provide a catalyst system which possesses economically practicable industrial oxidation rates, conversions and selectivities.
It is yet another object of this invention to eliminate the use of a phase transfer agent or surfactant in the reaction system.
It is a further object of this invention to obtain improved rates and selectivities in the olefin oxidation reaction by the use of a redox active metal component and/or the use of a ligand.
It is another object of this invention to be able to oxidize a large number of olefins which could not be oxidized efficiently previously because of one or more of the following problems: (a) isomerization, (b) chlorinated by-product formation, and (c) oxidation rates which are too low for industrial practice.